Point contacts for polysilicon emitter solar cell

ABSTRACT

The present invention relates to electrical contacts in a semiconductor device, and more particularly to methods and apparatuses for providing point contacts in a polysilicon emitter or HIT type solar cell. According to certain aspects, the invention uses a dielectric layer interposed between the substrate and a conductive layer to provide a limited area over which junction current can flow. The benefit is that the metal grid conductors do not need to align to the contacts, and can be applied freely without registration. Another benefit of the invention is that it provides increased efficiency for poly emitter and HIT cells through use of point contacts to increase current density. A further benefit is that patterning can be accomplished using low cost methods such as inclusion masking, screen printing or laser ablation. A still further benefit is that final contacts do not need alignment to the point contacts, eliminating registration required for conventional point contact designs.

CROSS-REFERENCE TO RELATED APPLICATIONS FIELD OF THE INVENTION

The present invention relates to electrical contacts in a semiconductordevice, and more particularly to methods and apparatuses for providingpoint contacts in a polysilicon emitter or HIT type solar cell.

BACKGROUND

It is known that a high efficiency solar cell can benefit from pointcontacts, as this increases the current density in the junctions.Examples include the PERL cell from the University of New South Wales,which achieved 24.7% efficiency, and the back contact cell made bySunPower Corporation of San Jose, Calif. However, such structures aredifficult to fabricate at low cost as they require several lithographicsteps that are registered to one another.

FIG. 1 shows one conventional junction contact structure. As shown inFIG. 1, a passivating oxide layer 104 is formed on a bulk material 100to minimize recombination on the surface. Holes are cut in the oxideusing photolithography and wet etching, and n(p)-type regions 102 arediffused into the p(n)-type substrate 100. A second registeredlithography is then performed to define contacts 106 over the contactholes. Therefore, at least two patterning steps are required, with thesecond registered to the first. Such patterning is difficult to performat the high throughput required for a solar cell line (on the order of3000 wafers per hour for 100 megawatts).

Accordingly, there remains a need in the art for a less complexstructure and technique for forming point contacts in a solar cell.

SUMMARY

The present invention relates to electrical contacts in a semiconductordevice, and more particularly to methods and apparatuses for providingpoint contacts in a polysilicon emitter or HIT type solar cell.According to certain aspects, the invention uses a dielectric layerinterposed between the substrate and a conductive layer to provide alimited area over which junction current can flow. The benefit is thatthe metal grid conductors do not need to align to the contacts, and canbe applied freely without registration. Another benefit of the inventionis that it provides increased efficiency for poly emitter and HIT cellsthrough use of point contacts to increase current density. A furtherbenefit is that patterning can be accomplished using low cost methodssuch as inclusion masking, screen printing or laser ablation. A stillfurther benefit is that final contacts do not need alignment to thepoint contacts, eliminating registration required for conventional pointcontact designs.

In furtherance of these and other aspects, a solar cell according toembodiments of the invention includes a conductive layer formed over asubstrate, the conductive layer providing for junction current flowbetween the underlying substrate and overlying conductors; a dielectriclayer between the conductive layer and the substrate that restricts thejunction current flow; and a plurality of point contacts formed in thedielectric layer that enables the junction current flow through thedielectric layer.

In additional furtherance of these and other aspects, a method offabricating solar cell according to embodiments of the inventionincludes forming a conductive layer over a substrate, the conductivelayer providing for junction current flow between the underlyingsubstrate and overlying conductors; forming a dielectric layer betweenthe conductive layer and the substrate that restricts the junctioncurrent flow; and forming a plurality of point contacts in thedielectric layer that enables the junction current flow through thedielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 shows a point contact emitter structure commonly used inconventional high efficiency solar cells.

FIG. 2 shows a solar cell structure for conventional HIT and polyemitter type solar cells.

FIGS. 3A to 3F illustrate an example process flow incorporatinginclusion masking according to embodiments of the invention.

FIGS. 4A to 4F illustrate an example process flow incorporating screenprinting according to embodiments of the invention.

FIGS. 5A to 5E illustrate an example process flow incorporating laserablation according to embodiments of the invention.

FIGS. 6A and 6B illustrate an example solar cell having point contactsin accordance with aspects of the invention.

DETAILED DESCRIPTION

The present invention will now be described in detail with reference tothe drawings, which are provided as illustrative examples of theinvention so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention to a single embodiment, butother embodiments are possible by way of interchange of some or all ofthe described or illustrated elements. Moreover, where certain elementsof the present invention can be partially or fully implemented usingknown components, only those portions of such known components that arenecessary for an understanding of the present invention will bedescribed, and detailed descriptions of other portions of such knowncomponents will be omitted so as not to obscure the invention. In thepresent specification, an embodiment showing a singular component shouldnot be considered limiting; rather, the invention is intended toencompass other embodiments including a plurality of the same component,and vice-versa, unless explicitly stated otherwise herein. Moreover,applicants do not intend for any term in the specification or claims tobe ascribed an uncommon or special meaning unless explicitly set forthas such. Further, the present invention encompasses present and futureknown equivalents to the known components referred to herein by way ofillustration.

The present inventors recognize that there exist certain cell structuresthat incorporate a conducting layer over the entire cell surface. Theseinclude the HIT cell (Sanyo Corporation) and the polysilicon emitter(PE) cell (See Green, Silicon Solar Cells, chapter 9). FIG. 2 shows howthe top structure of such cells are substantially similar. A thinpassivation layer such as amorphous silicon 202 a (HIT) or a tunnelingoxide 202 b (PE) is applied directly to the silicon 200 surface. Aconductor such as a transparent conductive oxide 204 a (TCO), as in thecase of the HIT cell, or doped polysilicon 204 b, as in the case of thePE cell, is applied over the passivation layer 202 a/202 b. Althoughthese cells benefit from improved contact passivation, they do not showadditional gains from higher current density that are possible withpoint contacts.

According to certain aspects, the present invention therefore uses adielectric layer interposed between the substrate and a conductive layerto provide a limited area over which junction current can flow, whichimproves current density. Further contrary to the prior art pointstructures and techniques, the metal grid conductors do not need toalign to the contacts of the present invention, and can be appliedfreely without registration.

FIGS. 3 to 5 show various ways in which a solar cell having pointcontacts can be made according to aspects of the invention. It should benoted that the principles of the invention can be applied to both HITand poly emitter solar cell structures. Accordingly, those skilled inthe art will appreciate that the contact passivating layer describedbelow can comprise amorphous silicon for the HIT cell and either atunnel oxide or direct contact for the PE cell, and the conductor layercan comprise a TCO for the HIT cell or doped polysilicon for the PEcell.

In any of the cell designs shown in FIGS. 3 to 5, a dielectric layer 302is first formed on the surface of the substrate (FIGS. 3A, 4A and 5A).This is preferably a thermal silicon dioxide, formed as either a rapidthermal oxide or as a conventional grown layer. This layer may berelatively thin, on the order of 50 to 150 Å thick, so that it does notinfluence the optical properties of the front surface. It should benoted that dielectric layer 302 preferably provides similar aspects of apassivating layer, such as the further purpose of reducing recombinationof carriers at the surface.

It is then necessary to form contact holes in the dielectric layer 302before completing the structure. Several different approaches are shownin FIGS. 3 to 5.

FIG. 3 shows a first embodiment of the invention, referred to herein asinclusion patterning. As shown in FIG. 3B, inclusions 306 are mixed intoa masking resist layer 304, which may be any material that resists thesubsequent etching of the dielectric layer. Such materials includelacquer films, photoresists, and chemical resins. Inclusions 306 may beparticles such as aluminum or calcium chloride. Particles thatcontaminate silicon or interface layers, such as sodium salts, are lessdesirable. The inclusions should be of a size on the order of thecontact holes, and much larger than the thickness of the resist, e.g.,10 μm inclusions for a 1 μm film. The inclusions 306 have the propertythat they dissolve or otherwise disrupt the masking action of theresist, allowing etching in their vicinity. Accordingly, as shown inFIG. 3C, when etching of the dielectric layer 302 is performed, holes308 are etched into the layer 302 in the vicinity of the inclusions 308.

While the inclusions are randomly dispersed in the resist material 304,they have a similar effect to a regular pattern when averaged over thecell area. Typical dimensions might be 10 μm openings on 50 μm spacings,for a 4% contact opening ratio, although other dimensions areacceptable. In general, the opening fraction of the surface area shouldnot exceed about 1%, or current crowding at the contacts will causeseries resistance losses.

In another embodiment of the invention shown in FIG. 4, a resist layer404 is screen printed on the surface and the resist is patterned usingconventional photolithography techniques to form pre-defined openings420, as shown in FIG. 4B. In the embodiments with a resist layer 420,the resist is removed after patterning using a solvent or resist stripsolution. Then, as shown in FIG. 4C, the dielectric layer is etchedthrough the patterned resist layer 404 to define contact holes 408.

In a third embodiment of the invention shown in FIG. 5, a laser is usedto cut holes directly in the dielectric layer 302. Accordingly, as shownin FIG. 5B, contact holes 508 are formed directly. In typicalembodiments, the laser damage must be etched away in an additionalprocess step as shown in FIG. 5C. In embodiments, a picosecond laser ispreferably used to perform the laser ablation as it tends to cause theleast laser damage.

Following any of the above processing shown in FIGS. 3 to 5, the contactholes 308/408/508 are then cleaned and the contact passivation layer andconductive layer 310 is deposited as shown in FIGS. 3E/4E/5D. It isfurther preferable to conduct a passivation anneal in forming gas afterdepositing the passivation layer and conductive layer 310.

Contacts 312 are then applied as shown in FIGS. 3F/4F/5E. In the case ofthe PE cell, an anti-reflection coating is typically needed as well.This is most simply applied as a deposition following the formation ofthe contacts.

Although FIGS. 3 to 5 show the contact holes 308 aligned with the gridcontacts 312, this is not necessary. In fact, one benefit of the methodof forming point contacts according to any embodiment of the inventionis that the metal grid conductors do not need to align to the contacts,and can be applied freely without registration. This aspect isillustrated in more detail in FIGS. 6A and 6B.

For example, FIG. 6A is a top view of an example solar cell 600 havingmetal conducting grid lines 312. It should be noted that the drawingsare intended to illustrate aspects of the invention, and are notnecessarily to scale. That said, an example solar cell may have gridlines about 80 μm wide and spaced about 2.5 mm apart. FIG. 6B provides amagnified view of a portion of solar cell 600. As shown in FIG. 6B,point contacts 308 formed according to embodiments of the invention canbe randomly dispersed with respect to grid lines 312. It should benoted, however, that contacts can also be regularly dispersed in otherembodiments. In any event, the grid lines need not be registered to thecontacts.

As mentioned above, typical dimensions of contacts 308/408/508 might beabout 10 μm wide openings with 50 μm spacings, for a 4% contact openingratio, although other dimensions are acceptable as will be appreciatedby those skilled in the art. In general, the opening fraction of thesurface area should not exceed about 1%, or current crowding at thecontacts will cause series resistance losses. Nevertheless, an aspect ofthe invention is that they provide a limited area over which junctioncurrent can flow, which improves current density. Although the presentinvention has been particularly described with reference to thepreferred embodiments thereof, it should be readily apparent to those ofordinary skill in the art that changes and modifications in the form anddetails may be made without departing from the spirit and scope of theinvention. It is intended that the appended claims encompass suchchanges and modifications.

1. A solar cell comprising: a conductive layer formed over a substrate,the conductive layer providing for junction current flow between theunderlying substrate and overlying conductors; a dielectric layerbetween the conductive layer and the substrate that restricts thejunction current flow; and a plurality of point contacts formed in thedielectric layer that enables the junction current flow through thedielectric layer.
 2. A solar cell according to claim 1 wherein thesubstrate is comprised of silicon.
 3. A solar cell according to claim 1wherein the point contacts are patterned using a masking layer andetching.
 4. A solar cell according to claim 1 wherein the point contactsare patterned using a laser.
 5. A solar cell according to claim 1wherein the dielectric layer with the point contacts formed thereincovers more than 50% of the area between the substrate and theconducting layer.
 6. A solar cell according to claim 2 wherein theconductive layer comprises a transparent conductor.
 7. A solar cellaccording to claim 2 wherein the conductive layer comprises dopedpolysilicon.
 8. A method of fabricating a solar cell comprising: forminga conductive layer over a substrate, the conductive layer providing forjunction current flow between the underlying substrate and overlyingconductors; forming a dielectric layer between the conductive layer andthe substrate that restricts the junction current flow; and forming aplurality of point contacts in the dielectric layer that enables thejunction current flow through the dielectric layer.
 9. A methodaccording to claim 8, wherein the step of forming the point contactsincludes: applying a resist layer that incorporates inclusions over thesubstrate; and etching the dielectric layer, wherein the dielectriclayer is etched predominately at the sites of the inclusions.
 10. Amethod according to claim 9 in which the inclusions dissociate in anetch bath.
 11. A method according to claim 9 wherein the inclusionscomprise aluminum particles.
 12. A method according to claim 8 whereinthe step of forming the point contacts includes patterning the pointcontacts using a laser.
 13. A method according to claim 8 wherein thestep of forming the point contacts is performed such that the dielectriclayer with the point contacts formed therein covers more than 50% of thearea between the substrate and the conducting layer.
 14. A methodaccording to claim 8 wherein the conductive layer comprises atransparent conductor.
 15. A method according to claim 8 wherein theconductive layer comprises doped polysilicon.